Currently, the most ubiquitous cellular communication system is the 2nd generation communication system known as the Global System for Mobile communication (GSM). Further description of the GSM TDMA communication system can be found in ‘The GSM System for Mobile Communications’ by Michel Mouly and Marie Bernadette Pautet, Bay Foreign Language Books, 1992, ISBN 2950719007.
3rd generation systems have recently been rolled out in many areas to further enhance the communication services provided to mobile users. One such system is the Universal Mobile Telecommunication System (UMTS), which is currently being deployed. Further description of CDMA and specifically of the Wideband CDMA (WCDMA) mode of UMTS can be found in ‘WCDMA for UMTS’, Harri Holma (editor), Antti Toskala (Editor), Wiley & Sons, 2001, ISBN 0471486876. The core network of UMTS is built on the use of SGSNs and GGSNs thereby providing commonality with GPRS.
In the future evolution of cellular communication systems, it is expected that these will increasingly be based on Internet Protocol (IP) traffic. For example, it is envisaged that a substantial part of the voice communication will be supported by Voice over IP (VoIP) traffic in the future. Accordingly, the 3rd Generation Partnership Project (3GPP), which is responsible for standardising the 3rd Generation cellular communication systems, has introduced a network architecture which supports IP traffic. This architecture is compatible with and supplements the traditional network architecture and is known as the IP Multimedia Subsystem (IMS).
The aim of IMS is not only to provide new services but to provide all the services, current and future, that the Internet provides. In addition, users have to be able to execute all their services when roaming as well as from their home networks. To achieve these goals, IMS uses open standard IP protocols, defined by the Internet Engineering Task Force (IETF). So, a multimedia session between two IMS users, between an IMS user and a user on the Internet, and between two users on the Internet is established using exactly the same protocol. Moreover, the interfaces for service developers are also based on IP protocols. This is why IMS truly merges the Internet with the cellular world; it uses cellular technologies to provide ubiquitous access and Internet technologies to provide appealing services.
In particular, IMS uses a VoIP implementation based on a 3GPP standardised implementation of the Session Initiation Protocol (SIP) and runs over the standard Internet Protocol (IP). Existing phone systems (both packet-switched and circuit-switched) are supported.
SIP is a standard for initiating, modifying, and terminating an interactive user session that involves multimedia elements such as video, voice, instant messaging, online games, and virtual reality. SIP is only used in setting up and tearing down voice or video calls. All voice/video communications are done over the Real-time Transport Protocol (RTP).
A goal for SIP was to provide a superset of the call processing functions and features present in the public switched telephone network (PSTN). As such, features that permit familiar telephone-like operations are present: dialing a number, causing a phone to ring, hearing ringback tones or a busy signal etc.
SIP also implements many more advanced call processing features. Furthermore, SIP is a peer-to-peer protocol. As such, it requires only a very simple (and thus highly scalable) core network with intelligence distributed to the network edge, embedded in endpoints (terminating devices built in either hardware or software). Many SIP features are implemented in the communicating endpoints.
IMS supports functionality for managing and controlling subscription information for the users of the system. Specifically, an IMS network comprises a Home Subscriber Server (HSS) which is a master user database that supports the IMS network entities that are actually handling the calls/sessions. These entities comprise the so-called Call Server Control Function (CSCF) elements. A CSCF also acts as a SIP Registrar and stores registration information (such public id, private id, contacts (the IP address of a device, capabilities)). It contains the subscription-related information (user profiles), performs authentication and authorization of the user, and can provide information about the physical location of user. An HSS may in many scenarios be considered to provide functionality equivalent to a GSM Home Location Register (HLR) and Authentication Center (AuC).
The IETF standards document RFC 3840 describes a mechanism which allows a network node to determine the capabilities of another subscriber. Specifically, the first node can transmit a SIP OPTIONS message to the Registrar Serving-CSCF (S-CSCF) with an identification of the public identity of the subscriber for which it is requesting information. In response, the S-CSCF (Registrar) retrieves the capability stored for that public identity (stored there at registration) and includes it in the SIP message 200OK which is then returned to the requesting node.
The subscriber may have a plurality of contacts (with different capabilities) or records associated with the same public identity. For example, the user may have different devices. SIP allows for the contact details of all the contacts to be provided in response to the OPTIONS message as well as the information of the capabilities of the contacts of the public identity. Thus, the mechanism described in RFC 3840 provides a way to let a node know about the capabilities of all the contacts belonging to a certain public identity.
However, the conventional SIP approach requires that the first node is aware of the public identity of the node(s) for which it is requesting information. Although this may be practical in some systems, it is disadvantageous in many cellular communication systems. Specifically, IMS allows a subscriber to have a plurality of public identities for example corresponding to different services and/or locations.
Furthermore, IMS provides another concept known as IMS subscription. This allows a group of subscribers to be linked to each other such that e.g. a common billing process is used for all subscribers of the group. For example, IMS allows the public identities of a family of subscribers to be linked and to have a single common billing process. However, in such systems, a node, such as a cellular user equipment intending to initiate a session with a specific subscriber, has to repeatedly transmit an OPTIONS message for each public identity in order to determine the capabilities available for contacting the subscriber (or group of subscribers). However, this is suboptimal and is in particular inconvenient, cumbersome, requires that all involved public identities are known by the requesting node, results in high levels of signaling and is resource demanding.
Hence, an improved system for obtaining information of subscribers would be advantageous.